The invention relates to a process for the production of carbon nanotubes in agglomerated form and novel carbon nanotube agglomerates obtainable therefrom.
According to the prior art, carbon nanotubes are understood as being chiefly cylindrical carbon tubes having a diameter of from 3 to 100 nm and a length which is a multiple of the diameter. These tubes comprise one or more layers of ordered carbon atoms and have a core of different morphology. These carbon nanotubes are also called, for example, “carbon fibrils” or “hollow carbon fibres”.
Carbon nanotubes have been known for a long time in the technical literature. Although Iijima (publication: S. Iijima, Nature 354, 56-58, 1991) is generally named as the discoverer of carbon nanotubes, these materials, in particular fibrous graphite materials with several layers of graphite, have already been known since the 1970s or early 1980s. Tates and Baker (GB 1469930A1, 1977 and EP 56004 A2) described for the first time the deposition of very fine fibrous carbon from the catalytic decomposition of hydrocarbons. Nevertheless, the carbon filaments produced on the basis of short-chain hydrocarbons are not characterized in more detail with respect to their diameter.
Conventional structures of these carbon nanotubes are those of the cylinder type. In the case of cylindrical structures, a distinction is made between the single-wall mono-carbon nanotubes (single wall carbon nano tubes) and the multi-wall cylindrical carbon nanotubes (multi wall carbon nano tubes). The usual processes for their production are e.g. arc processes (arc discharge), laser ablation, chemical deposition from the vapour phase (CVD process) and catalytic chemical deposition from the vapour phase (CCVD process).
Iijima, Nature 354, 1991, 56-8 discloses the formation of carbon tubes in the arc discharge process which comprise two or more layers of graphene and are rolled up to a seamless closed cylinder and nested in one another. Depending on the rolling up vector, chiral and achiral arrangements of the carbon atoms in relation to the longitudinal axis of the carbon fibres are possible.
Carbon nanotubes which have a so-called fishbone morphology are moreover described (J. W. Geus, EP application 198,558) and again others which have a bamboo-like structure (Z. Ren, U.S. Pat. No. 6,911,260 B2).
Structures of carbon tubes in which an individual cohesive layer of graphene (so-called scroll type) or interrupted layer of graphene (so-called onion type) is the basis of the structure of the nanotubes were described for the first time by Bacon et al., J. Appl. Phys. 34, 1960, 283-90. The structure is called scroll type. Corresponding structures were also found later by Zhou et al., Science, 263, 1994, 1744-47 and by Lavin et al., Carbon 40, 2002, 1123-30.
A further type of scroll structures was described recently in the patent application WO2009036877 A2. These CNT structures comprise several layers of graphene which are present combined into a stack and rolled up (multiscroll type). The individual layers of graphene or graphite in these carbon nanotubes, seen in cross-section, run continuously from the centre of the CNT to the outer edge without interruption.
In the context of the invention, all the carbon nanotube structures described above are summarized in the following simply as carbon nanotubes, fibrils or CNT or MWCNT.
The methods now known for the production of carbon nanotubes include arc discharge, laser ablation and catalytic processes. In many of these processes carbon black, amorphous carbon and fibres of high diameter are formed as by-products. In the case of the catalytic processes, a distinction may be made between the deposition on e.g. supported catalyst particles and the deposition on metal centres formed in situ with diameters in the nanometer range (so-called flow process). In the production via the catalytic deposition of carbon from hydrocarbons which are gaseous under the reaction conditions (CCVD; catalytic carbon vapour deposition in the following), acetylene, methane, ethane, ethylene, butane, butene, butadiene, benzene, toluene, xylene and further carbon-containing educts are mentioned as possible carbon donors. CNTs obtainable from catalytic processes are therefore preferably employed.
The catalysts as a rule contain metals, metal oxides or decomposable or reducible metal components. For example, Fe, Mo, Ni, V, Mn, Sn, Co, Cu and further sub-group elements are mentioned as metals for the catalyst in the prior art. The individual metals indeed usually have a tendency to assist in the formation of carbon nanotubes, although according to the prior art high yields and low contents of amorphous carbons are advantageously achieved with those metal catalysts which are based on a combination of the abovementioned metals. CNTs obtainable using mixed catalysts are consequently preferably to be employed.
Particularly advantageous catalyst systems for the production of CNTs are based on combinations of metals or metal compounds which contain two or more elements from the series consisting of Fe, Co, Mn, Mo and Ni.
From experience, the formation of carbon nanotubes and the properties of the tubes formed depend in a complex manner on the metal component used as the catalyst or a combination of several metal components, the catalyst support material optionally used and the interaction between the catalyst and support, the educt gas and its partial pressure, an admixing of hydrogen or further gases, the reaction temperature and the dwell time or the reactor used.
WO 2006/050903 A2 discloses a process which is particularly preferably to be employed for the production of carbon nanotubes.
In the various processes mentioned so far employing various catalyst systems, carbon nanotubes of different structures, which can be removed from the process predominantly as carbon nanotube powder, are produced.
Carbon nanotubes which are further preferably suitable for the invention are obtained by processes which are described in principle in the following literature references:
The production of carbon nanotubes with diameters of less than 100 nm is described for the first time in EP 205 556 B1. For the production, light (i.e. short- and medium-chain aliphatic or mono- or dinuclear aromatic) hydrocarbons and a catalyst based on iron, on which carbon carrier compounds are decomposed at a temperature above 800-900° C., are employed here. A detailed description of the agglomerate morphology of the carbon nanotubes has not been given.
WO86/03455A1 describes the production of carbon filaments which have a cylindrical structure with a constant diameter of from 3.5 to 70 nm, an aspect ratio (ratio of length to diameter) of greater than 100 and a core region. These fibrils comprise many continuous layers of ordered carbon atoms which are arranged concentrically around the cylindrical axis of the fibrils. These cylinder-like carbon nanotubes were produced by a CVD process from carbon-containing compounds by means of a metal-containing particle at a temperature of between 850° C. and 1,200° C. The catalyst for this reaction was obtained by impregnation of various aluminium oxides with iron salts in aqueous solution. The aluminium oxides were partly calcined under oxidative conditions at temperatures of up to 1,100° C. before the loading with iron salts and up to 500° C. after the loading. A reductive calcining of the aluminium oxide supports loaded with iron up to 1,100° C. was also investigated. The fibrils produced were examined under a microscope, although no information is given on fibril agglomerates or the morphology thereof. Moy and colleagues (U.S. Pat. No. 5,726,116, U.S. Pat. No. 7,198,772 B2; Hyperion Catalysis International Inc.) report for the first time on various fibril agglomerate morphologies which form according to the catalyst support chosen. In this context, Moy distinguishes between 3 morphologies, the bird's nest structure (BN) the combed yarn structure (CY) and the open net structure (ON). In the bird's nest structure (BN), the fibrils are arranged randomly tangled in a form such that a ball of fibrils intertwined with one another similar to the structure of a bird's nest is formed. This structure can be obtained e.g. by employing aluminium oxide as the support material for the iron/molybdenum catalyst.
The combed yarn structure (CY) comprises bundles of carbon nanotubes which mostly have the same orientation relative to one another. The open net structure (ON) is formed by fibril agglomerates in which the fibrils are loosely woven with one another. These two structures are formed when gamma-aluminium oxide, e.g. ALCOA H705, or magnesium oxide (Martin Marietta) is used as the support material with precipitated or impregnated catalysts. The descriptions of the morphologies contain no more precise information with respect to the size of the agglomerates, definitions of the alignments of the CNT within the agglomerates or further physical or geometric parameters for characterization of the structures. The agglomerates formed from CY and ON structures are said to be more easily dispersible than those of the BN structure.
Another method for the production of CNT catalysts is co-precipitation of metal compounds, e.g. of oxides, from a solution. Spherical particles of mixed metal oxides are formed from this simultaneous precipitation. In contrast to the supported catalyst systems described above, in which the active metal is to be found only on the surface of an (inert) support substance, in the case of the co-precipitated spherical mixed oxides the catalytic active metal is distributed everywhere within the catalyst particle homogeneously with the other metal oxides. The loading with active metal and therefore the efficiency is increased. The catalytically inactive metal oxides function here as binders and spacers. In the ideal case, this catalyst is broken open completely during the reaction and all the active metal centres become accessible for the reaction. The original catalyst particle is completely destroyed in this process. Moy et al. (U.S. Pat. No. 6,294,144; U.S. Pat. No. 7,198,772) investigated systematically co-precipitated catalysts based on iron, molybdenum and aluminium oxide for the synthesis of carbon nanotubes and in all cases obtained CNT agglomerates with a bird's nest structure (BN).
The patent application WO 2009 043445 A2 describes a co-precipitated catalyst based on a mixed oxide of cobalt, manganese, aluminium and magnesium oxide which is suitable for the production of carbon nanotubes and is distinguished by a very high efficiency. The carbon nanotube agglomerates obtained in this way are distinguished by a high degree of tangling, similarly to the CNT with a bird's nest structure, in which the individual CNT are woven with one another without alignment. The dispersing of these agglomerates, e.g. in polymers or low-viscosity systems, such as solvents, is made difficult as a result. The forces necessary for good dispersing also lead, in addition to breaking up of the CNT agglomerates, to undesirable damage to the individual CNT (e.g. shortening) and the polymer (reduction in the molecular weight).
It is desirable to have a process for the production of CNT agglomerates in which the catalyst delivers high conversions and CNT yields and the product is to be simultaneously dispersed in polymers (thermoplastics) easily and with a low introduction of energy and force, in order to avoid damage both to the individual CNT and to the polymer during breaking up of the agglomerate.